It is well known that a credit card includes a magnetic stripe which has encoded on it various information identifying the individual to whom the card is issued, the issuing bank, and other related information. Information is stored on such a stripe in a "Biphase" mark coding technique in "cells" defined by a leading and an associated trailing magnetic flux reversal, the spacing between which defines the period or length of the cell. A binary one is defined by an additional flux reversal in the middle of a cell; a binary zero is defined by the absence of an additional flux reversal. Thus, the information bits (data) on a magnetic stripe are represented by a sequence of binary ones and zeros so defined.
In most applications, a card with a magnetic stripe with recorded data is passed by a magnetic read head where each magnetic flux reversal is converted into an electrical signal. The standard densities at which the bits of data occur are 75 and 210 bits per inch (BPI). The electronic circuitry responsive to the signals detected by the read head are designed for either 75 or 210 BPI data densities but not both. The reason that such circuitry is not responsive to signals at both signal densities is that a certain percentage of magnetic stripe cards in circulation contain defects which produce spurious read head currents which lead to erroneous signals. Further, frequently, scratches occur on the magnetic stripe through contact with, for example, keys. These defects and scratches not only produce spurious signals but those spurious signals seem to occur at about the 210 BPI rate because they tend to occur close together separated by distances small compared to the cell size.
In order to avoid errors due to such defects and scratches, existing magnetic stripe decoders at 75 BPI densities include a filter circuit which suppresses responses to signals which occur above 75 BPI. Decoders adapted for operation at 210 BPI have less of a need for such filters. Errors do occur but less frequently at 210 BPI. Thus two different types of decoders for magnetic stripe sensing are available for operation at two distinct bit densities but not both.
It would, of course, be advantageous to produce a single decoder which could respond to any bit density of from about 75 BPI to 210 BPI and still avoid spurious signals.